1. Field of the Invention
The present invention relates to a plasma processing apparatus and, more particularly, to a plasma processing apparatus having an electrode connected to a DC power source.
2. Description of the Related Art
There is known a parallel plate type plasma processing apparatus provided with an accommodating compartment for accommodating a wafer serving as a substrate, a lower electrode disposed within the accommodating compartment and connected to a high-frequency power source, and an upper electrode disposed so as to face the lower electrode. In this plasma processing apparatus, a processing gas is introduced into the accommodating compartment and high-frequency power is supplied into the accommodating compartment. In addition, plasma is produced from the introduced processing gas by the high-frequency power when the wafer is accommodated into the accommodating compartment and mounted on the lower electrode. Thus, a plasma treatment, such as an etching treatment, is performed on the wafer using the plasma, etc.
In recent years, there has been developed a plasma processing apparatus wherein an upper electrode is connected to a DC power source and a DC voltage is applied to the inside of the accommodating compartment, in order to improve the performance of plasma treatments. In order to apply a DC voltage to the inside of the accommodating compartment, there is the need to provide a ground electrode used for the DC voltage applied to the inside of the accommodating compartment within which a surface of the ground electrode is exposed (hereinafter simply referred to as the “ground electrode”). However, when performing a plasma treatment using a reactive processing gas, a reaction product (deposition) may adhere to a surface of the ground electrode and, therefore, a deposition film may be formed.
Since the deposition film is insulative, the flow of a DC current from the upper electrode to the ground electrode is blocked, thereby disabling the application of a DC voltage to the inside of the accommodating compartment. As a result, plasma within the accommodating compartment may fall into an unstable state or plasma treatment characteristics may change.
In consideration of the above, the present inventor has gained the knowledge, through experiments, that a main contributor to the formation of a deposition film is positive ions in plasma and that there is only a small quantity of positive ions in the vicinity of corners formed by component parts of a plasma processing apparatus. Based on the knowledge, the present inventor has proposed preventing a deposition film or the like from being formed on a surface of the ground electrode by disposing a ground electrode in the vicinity of the corners (for example, see Japanese Patent Application No. 2006-081352).
However, since the ground electrode remains exposed within the accommodating compartment even if disposed in the vicinity of the corners, some positive ions reach the ground electrode and form a deposition film on a surface thereof. The deposition film is formed slowly and, therefore, DC voltage application to the inside of the accommodating compartment is not immediately disabled. However, it has been confirmed by the present inventor that if the total time of a plasma treatment exceeds, for example, 70 hours, the value of a DC current flowing within the accommodating compartment decreases to 1.43 A to 1.33 A. Thus, there is the problem that a decrease in the value of a DC current causes a change in plasma treatment characteristics.